In the mining industry, it is common for mined materials such as coal, oil sand, etc. to contain a certain amount of metallic scrap such as bucket teeth, crusher teeth, tools, etc. (commonly referred to as “tramp metal”) that can cause damage to upstream equipment. Oil sand is a type of bitumen deposit typically containing sand, water and very viscous oil (the bitumen). When the oil sand deposit is located relatively close below the ground surface, the oil sand is often extracted from the deposit by mining. The oil sand is mined by excavating down through the ground surface to where the oil sand deposit occurs and removing oil sand from the deposit with heavy machinery.
Typically, this removal of the oil sand from the deposit is done with some of the largest power shovels and dump trucks in the world, with the power shovels removing shovel-loads of oil sand from the deposit and loading the collected oil sand onto conveyors to be carried away for further processing.
The viscous bitumen tends to hold the sand and water together causing the mined oil sand to contain lumps and chunks, some of which can be quite large. Because of the size of some of these pieces of mined oil sand, the mined oil sand is typically “pre-crushed” by running it through a preliminary crusher to crush the pieces of oil sand to a suitable size for transport on a conveyor (i.e. conveyable size).
The pre-crushed oil sand is then transported by conveyor to a slurry preparation unit as known in the art where the pre-crushed oil sand is further processed to form an oil sand and water slurry. One example of a slurry preparation unit is described in Canadian Patent Application No. 2,480,122, which unit comprises a series of roll crushers spread vertically throughout a portion of a slurry preparation tower. The slurry preparation tower typically uses gravity to move the oil sand through the tower. Typically, each roll crusher is made up of a number of crusher rolls spaced a set distance apart to reduce the size of large pieces of oil sand before the pieces of oil sand drop through the crusher rolls to the next roller crusher beneath or the bottom of the slurry preparation tower. Each successively lower roll crusher reduces the pieces of oil sand even smaller until the oil sand is fine enough to form a pumpable oil sand slurry.
At the same time the oil sand is passing though the different roll crushers, heated water is added to the oil sand to form it into a slurry. Typically, the stream of oil sand passing through the levels of roll crushers is sprayed with the heated water, as it passes down the tower. The mixing of this oil sand with the streams of hot water will form the eventual oil sand slurry, which is typically received in a pump box for feeding the slurry to a pump and pipeline system.
As long as only pre-crushed oil sand is being fed into a slurry preparation unit such as the aforedescribed slurry preparation tower, the slurry preparation unit operates properly. However, problems can occur when a piece of sizable metal (commonly called tramp metal) is present in the pre-crushed oil sand traveling along the conveyor. This tramp metal is often a piece of metal from machinery used earlier in the process, such as a piece of shovel tooth from the power shovel or a piece of crusher tooth from the primary crusher. If this piece of tramp metal is large enough, when it is fed into the slurry preparation tower along with a portion of oil sand, the tramp metal can damage or even jam one of the roll crushers used in the slurry preparation tower. With the roll crushers damaged or jammed, the entire process has to be stopped while the crusher rolls are either repaired or the jam is located and the tramp metal removed. This can lead to lengthy outages to remove the object from the crusher rolls and affect repairs if any damage has occurred.
Unfortunately, this inclusion of tramp metal in the pre-crushed oil sand often occurs quite frequently, with occurrences of tramp metal in a flow of pre-crushed oil sand having been seen as frequently as once per 12 hours shift.
Previously a complex system of screens has been used to locate and remove this tramp metal from the process. However, these systems greatly complicated the process because they added a number of additional steps that could limit the amount of oil sand that was processed. Additionally, because of the conditions they were operating under, the screens often had relatively low operation lives, requiring frequent repairs and replacements. Most modern processes have completely removed the screens from the system and instead rely on metal detectors to locate pieces of tramp metal in the oil sand.
Metal detectors are now commonly used to locate tramp metal in the flow of pre-crushed oil sand along a conveyor. When the metal detector detects a piece of tramp metal in the oil sand, the metal detector either alerts an operator that metal has been detected in the flow of pre-crushed oil sand or sends a signal stopping the conveyor and preventing the tramp metal from being fed into the slurry preparation tower. Once the conveyor is stopped, someone is sent out to locate the tramp metal and remove it from the pre-crushed oil sand.
However, the detection of tramp metals in the flow of oil sand is far simpler than the eventual locating and removal of the tramp metal from the oil sand once the conveyor is stopped. The oil sand on the conveyor can be 1-2 feet in depth, burying the often relatively small tramp metal. Additionally, because of the delay in time between the receipt of the alert from the metal detector and the stopping of the conveyor, the tramp metal will often vary in distance downstream from the metal detector, making it guess work for a person to figure out where along the length of pre-crushed oil sand the tramp metal lies. The conveyor carrying the oil sand can be hundreds of meters long or more, requiring a conveyor belt twice as long as the distance covered by the conveyor. During operation the conveyor belt is commonly driven at speeds between 3-4 meters per second. The significant weight of the belt, as well as its speed, results in the moving belt having significant inertia often requiring substantial force and a significant period of time for the conveyor belt to be decelerated and stopped. This can make the estimating of the position of the tramp metal buried in the oil sand on the belt less than precise for the human operators. Additionally, there are numerous factors with the conveyor, such as wear on bearing and the engine driving the conveyor belt, that can make the deceleration time to stop the belt vary over the life of the conveyor.
Not only does it take time to decelerate and halt the conveyor and then restart and accelerate the conveyor back up to the desired operating speed, because of the force required to decelerate and accelerate the conveyor, frequently stopping the conveyor can increase the wear on the conveyor and its components, impacting the lifespan of the conveyor.
Additionally, the affects of halting the conveyor and stopping the flow of oil sand into the slurry preparation tower are not as simple as temporarily delaying the process. The processing of oil sand is commonly done as a continuous process. Stopping the conveyor can not only affect all later steps of the process, it can also affect the quality of the formed slurry. The slurry preparation tower requires a relatively consistent feed rate of oil sand to result in a high quality oil sand slurry having a consistent density. It is known that conditioning of oil sand slurry (e.g., release of bitumen flecks, attachment of bitumen flecks to air bubbles, etc.) is most efficient within a relatively narrow density range resulting from a proper ratio of oil sand to water in the slurry. Interrupting the supply of particulate oil sand to the slurry preparation tower can reduce the quality of the slurry, reducing the effectiveness of later process steps or even rendering a slurry unusable. In addition to the interruption, the time needed for the deceleration of the conveyor when the conveyor is being stopped to remove the tramp metal can result in oil sand slurry with a diminishing density as the flow rate of oil sand entering the slurry preparation tower decreases with the deceleration of the conveyor. When the conveyor is being sped up again, the time needed to accelerate the conveyor up to speed can also result in variations in the density of the resulting slurry.
There is therefore a need to remove pieces of tramp metal from a flow of particulate material such as oil sand being moved on a conveyor without halting the flow of same for a significant period of time.